Ratification of spreadsheet for utility charges to tenants

I own a HMO property occupied by 6 individual tenants. Each tenant is to pay 1/6th of the electric and gas costs after I make a contribution of £100 per month.

In order to track and calculate what each tenant owes, I have created a spreadsheet that details the meter readings and utilities charges on an ongoing basis. It takes into account the various different tariffs incurred across a period of a few years, so is slightly complex,

Each tenant gets a copy of the spreadsheet tailored to their time in occupation, which calculates what they owe.

Three tenants are refusing to pay. They have questioned the accuracy of the spreadsheet - rightly on a couple of occasions, but now they do not trust its accuracy.

I have run the spreadsheet through chat gpt and Claude, along with copy utility bills, and both tools have confirmed the spreadsheets are accurate.

I now need a chartered or certified accountant to run a final check and confirm their approval so that I can finally enforce payment and if necessary take the matter to court. Obtaining this certification/approval will remove the doubts as to accuracy and enable me to recover payment.